Electromechanical liquid level servo gauges (ESGs) are used for the accurate measurement of product level and the water interface level in bulk storage tanks used for typical hydrocarbons (often referred to as fuel and oil) and a variety of other liquid chemicals. These products range from very light chemicals, like so called LPG's (mixtures of propane and butane or even liquefied natural gas (LNG)) to all types of refined products such as naphtha, gasoline, diesel, jet fuels, lubricants and all types of chemicals, both pure and mixed.
The servo principle is based on the measurement of the apparent weight of a displacer that is within the tank. The displacer is a mechanical body suspended on a strong thin measuring wire, where the displacer material has a higher density than the liquid to be measured. The measurement wire is wound on a high accuracy machined grooved drum with a calibrated circumference. The apparent weight resulting from the weight of the displacer minus the weight of the displaced liquid product is measured and is then used by a computing device such as a microcontroller with the servo motor used to rotate drum in order to position the displacer at a different height in the tank.
By rotating the drum the wire is spooled up or paid out into the tank and the displacer is raised or lowered until the measured apparent weight equals the programmed set point. For safety reasons typically a magnetic coupling (using pole pairs) may be located between drum and electronics (motor, microcontroller, electronics, etc.) as many of the liquids products which are commonly stored in bulk storage tanks are flammable and typically need an explosion safe design. The displacer being more dense than the density of the product in the tank is basically kept at the same level using Archimedes law which indicates that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces.
The apparent weight resulting from the displaced liquid is dependent on the density of the displaced liquid and the amount of the displaced liquid. The amount of the displaced liquid depends again on the shape of the displacer, and the set point (i.e. how much weight there needs to be displaced).
Vapor influence, caused by dense vapors, especially on products with low dielectric constant and a relative high dipole moment result in accuracy limiting physics, which make radar unsuitable and unacceptable for legal metrology use. The large variation in saturation which are not predictable also makes it generally not possible to compensate for these vapor effects, which especially occur with light hydrocarbons and chemicals, where ESGs do not have these limitations. Some examples are LPGs, ethanol and multiple industrial solvents. Also foam is an example where an ESG still can detect the liquid surface while radar will generally not find any reflection. This means that ESGs are still an important and much relied upon accurate measurement technology, especially when high and certified accuracy is a needed, such as for custody transfer applications.